lab_inheritance Insidious Inheritance

Assignment Description

In this lab you will get experience with some of the implementation issues and
conceptual details of inheritance. Inheritance is a mechanism for increasing
the reusability and reliability of C++ code. It is worth mentioning that
inheritance is a characteristic of all object oriented programming languages.
Our goal is to give you a glimpse of the functionality of inheritance, so that
you can make informed design decisions in the future. Please read through the
entire lab before you begin. The compilation notes at the bottom will tell you
how to organize your development files.

Lab Insight

This lab teaches some of the characteristics observed in object oriented programming. These characteristics are useful in software design when building code bases for large projects, APIs, and production code bases. Some classes that further extend on these concepts include CS 427, CS 242, and CS 422. This lab will help you build clean code bases where the OOP (Object-Oriented Programming) characteristics help minimize having to rewrite redundant code as well as make use of virtual inheritance to treat custom sub-classes the same way as their base classes without losing the custom functionality of these subclasses.

Upon a successful merge, your lab_inheritance files are now in your lab_inheritance directory.

The code for this activity resides in the lab_inheritance/ directory. Get
there by typing this in your working directory:

cd lab_inheritance/

You will only need to modify the following files:

shape.{cpp,h}

circle.{cpp,h}

truck.{cpp,h}

flower.{cpp,h}

drawable.h

Class Hierarchy

To help us understand class hierarchies better here is an example of a simple
class hierarchy showing that a Dogis aAnimal.

The code would look something like the following:

classAnimal {
public:
string name;
virtualvoid speak() = 0;
/* The = 0 at the end of the method means that the method is a pure virtual method
* meaning that it does not have an implementation and it delegates the task
* of implementing the method to the classes that is derived from it */
};
classDog : public Animal {
public:
string breed;
/* Dog inherits speak from Animal */void speak();
};
void Dog::speak() {
cout << "Woof Woof" << endl;
}

In this example Animals have a name and can speak but since speak is a
pure virtual method we CANNOT construct an Animal by itself. That is Animal
is an abstract class and it can only be constructed by one of its derived
classes. For example, a Dog is a derived class of Animal. This means that a
Dog is aAnimal, and, therefore, it inherits a name and a speak
method from Animal. However, since the Animal’s speak does not have an
implementation, Dogmust implement the speak method.

Here is an example of how we could use a Dog object:

Dog* d = new Dog();
/* Like usual we can access all the public methods and member variables of a
* Dog */
d->breed;
/* But now since a Dog is an Animal we can also do this too */
d->name; // inherited from Animal
d->speak(); // inherited from Animal and since it is a Dog speak() will print
// "Woof Woof"
/* Additionally we can treat our Dog only like an Animal like this */
Animal* a = d;
/* But now we can only do the following */
a->name;
a->speak(); // Still prints "Woof Woof" because speak is a virtual method.
a->breed // ERROR! This will NOT work since we perceive it as an Animal now
/* Additionally, if we try to have our Animal pointer point back to a Dog
* pointer this will cause a problem because an Animal Is NOT A Dog. */
Dog* d2 = a; // ERROR! Animal Is NOT A Dog
/* Furthermore, since Animal is abstract and has a pure virtual method
* we CANNOT construct one! */
Animal a2; // ERROR! Animal is an abstract class

Now that we can understand a simple class hierarchy, let’s look at a more
complex one. Here is a diagram depicting the class hierarchy that is used in
this lab. (Note: This diagram is missing some information, e.g. methods, member
variables, etc.., for demonstration purposes)

This means everything is a Drawable and will have a draw method. Code like
the following is perfectly acceptable:

Drawable* triangle = new Triangle(....);
Drawable* circle = new Circle(...);
Drawable* rectangle = new Rectangle(....);
Drawable* truck = new Truck(...);
Drawable* flower = new Flower(....);
/* Now the only thing we can use on triangle, circle, rectangle, truck, and
* flower is draw but what gets drawn will change depending on what type the
* pointer is actually pointing to. This is called polymorphism, the behavior
* changes depending on the actual type of the object being pointed to. */
PNG canvas;
triangle->draw(&canvas); // draws a Triangle even though triangle is a Drawable*
circle->draw(&canvas); // draws a Circle even though circle is a Drawable*
rectangle->draw(&canvas); // draws a Rectangle even though rectangle is a Drawable*
truck->draw(&canvas); // draws a Truck even though truck is a Drawable*
flower->draw(&canvas); // draws a Flower even though flower is a Drawable*

Look at main.cpp for a working example executable in action. main.cpp gets
compiled and linked into an executable named lab_inheritance. Follow the
instructions below to build, run, and view the output:

The Makefile provided for this MP will create two useful executables when you
run make. It will generate lab_inheritance and lab_inheritance-asan. So
when you want to test a specific part of your MP you can use either of those.
For example when you run

./lab_inheritance

You could also run it as:

./lab_inheritance-asan

if you want to test it with Address Sanitizer, and to check for memory leaks:

./lab_inheritance-asan

Additionally you’re free to run Valgrind on the normal executable:

valgrind --leak-check=full ./lab_inheritance

This lab will use all of these objects in interesting ways but as you will
quickly see they are not working the way the should. Your objective for this
lab is to go through the 5 test executables and fix the code to work correctly
by modifying how the classes in the hierarchy declare and implement their
methods.

Once you have fixed all the Valgrind errors, you can test your program output
following the directions below.

Exercise 1: Fix the Virtual Methods

Please build and run test_virtual:

make test_virtual test_virtual-asan # make test_virtual
./test_virtual-asan # run test_virtual with asan
valgrind ./test_virtual # run test_virtual with valgrind

As you will see when you run test_virtual, the output will say:

The Perimeters are NOT the same.
The Areas are NOT the same.

However, if you look closely at the code they should be the same because both
of the pointers in test_virtual.cpp point to the same object!

Exercise

Investigate and fix the code so that the areas and the perimeters are the
same.

To fix this problem you should only need to modify shape.cpp and/or
shape.h.

Exercise 2: Fix the Destructor

Please build and run test_destructor:

make test_destructor test_destructor-asan # make test_destructor
valgrind --leak-check=full ./test_destructor # run test_destructor in valgrind
./test_destructor-asan # test it with Address Sanitizer and check for leaks

When you run test_destructor in Valgrind or ASAN you will see that
test_destructor is leaking memory. However, if you look closely, Triangle
does have a valid destructor and it is being called in test_destructor!

Exercise

Investigate and fix the code so that the there is no more memory leak inside
of test_destructor.

To fix this problem you should only need to modify drawable.h and
shape.h.

Exercise 3: Fix the Constructor

Please build and run test_constructor:

make test_constructor # make test_constructor
./test_constructor # run test_constructor

When you run test_constructor you will see the following output:

Circle's color is NOT correct!
Circle's center is NOT correct!

If you look closely, we are constructing a Circle with a valid center and
color. However, when it is being drawn and when we ask for the Circle’s
center and color they are not the same!

Exercise

Investigate and fix the code so that the Circle is being constructed with
the proper center and color.

To fix this problem you should only need to modify circle.cpp

The correct test_constructor.png should look like the following:

Exercise 4: Fix the Pure Virtual Method

Please build and run test_pure_virtual.

make test_pure_virtual # make test_pure_virtual
./test_pure_virtual # run test_pure_virtual

When you try to make test_pure_virtual you will see that it does not compile.

However, if you look at the truck.{h,cpp} it is a fully featured class! Why
is it not compiling?

Exercise

Investigate and fix the code so that test_pure_virtual compiles, runs, and
outputs a Truck.

To fix this problem you should only need to modify truck.h and truck.cpp.

In order to have the Truck draw properly you will first need to have
Exercise 3 completed.

The correct test_pure_virtual.png should look like the following:

Exercise 5: Fix the Slicing

Please build and run test_slicing with:

make test_slicing # make test_slicing
./test_slicing # run test_slicing

After you run test_slicing open up its output test_slicing.png. You will
see that a Flower has NOT been drawn. For some reason just a bunch of X’s has
been drawn and a red circle.

If you look at flower.h and flower.cpp, we have all of the proper member
variables set up. However, when we try to draw them they are drawn incorrectly.

Exercise

Investigate and fix the code so that test_slicing outputs a Flower.

To fix this problem you should only need to modify flower.h and
flower.cpp.

You must use polymorphism!

The correct test_slicing.png output should look like the following:

Testing Your Code

Run the Catch tests as follows (this requires your code to compile when run simply as make):

make test
./test

Cleaning up files

To clean up your working repository and remove the test images produced by your program, you can type the following command:

make tidy

Submitting Your Work

The following files are used to grade this assignment:

shape.cpp

shape.h

circle.cpp

circle.h

truck.cpp

truck.h

flower.cpp

flower.h

drawable.h

All other files including any testing files you have added will not be used for grading.